Internal combustion engines, whether of Diesel cycle, Otto cycle, two or three stroke, comprise at least one sliding element such as a piston ring.
In this respect, the piston ring acts in the sealing of the space between the cylinder liner and the body of the piston, isolating the combustion chamber from the other internal components of the engines. The piston ring is disposed radially upon the base of the body of the piston, preventing the combustion gases from escaping from the combustion chamber towards the sump and preventing the engine oil from penetrating into the combustion chamber.
Some internal combustion engines, principally engines operating with the Diesel cycle, work with high loads. Other examples may be found in high performance petrol engines. Independently of the examples which may be cited, there is a trend for engines to operate at high speed with high power, reduced clearances and, consequently, subject to a severe tribological regime. Such conditions are naturally more exacting upon the mechanical components thereof. In this respect, the rings utilized in these high power or performance engines require low friction, high hardness and high wear resistance.
In addition, it is important to stress that the environmental impact of internal combustion engines, when taken together with the great need for performance and durability, results in a general manner in the need to work with tighter tolerances which, naturally, is translated into the utilization of increasingly thin layers of lubricating oils. As will be seen hereinafter, the present invention has been specifically developed to offer a solution the excellent performance whereof flows from the utilization of predominantly less viscous oils.
The piston rings of the state of the art generally comprise coatings of hard amorphous carbon, also known as DLC (diamond like carbon) or hydrogenated DLC nanostructure free of hydrogen, as a solution for achieving low friction and high wear resistance.
In general, the solutions of the state of the art apply the coating of DLC in a composition incorporating sp3 and sp2 bonds. Nevertheless, it must be noted that by virtue of the high dimensional stability of the sp3 bonds, the hardness thereof exceeds that found with the sp2 bonds. For this reason, diverse solutions of the state of the art make use of a DLC layer of sp3 (diamond type) coated with a thin layer of sp2 (graphite type).
For a better understanding, the sp2 layer, by virtue of being very soft and lubricant, is utilized to permit the bedding in of the ring within the cylinder liner and preventing the sp3 layer from entering directly into contact with the cylinder liner. Such concern is extremely valid, by virtue of the fact that, in addition to the fact that the sp3 layer may score the cylinder liner, the concentration of stresses present in the sp3 layer is very high such that it may generate disastrous consequences both for the cylinder liner, scoring it, and for the coating, wherein cracks and the propagation thereof may occur, consequently leading to the diminution in the working life of the engine.
The patent document WO 2010133633 reveals a piston ring provided with a DLC coating of the ta-C type having a thickness exceeding 10 micrometers and having at least one residual stress gradient to endow durability and low friction upon the component. It is to be noted that the high internal stresses inherent in the deposition of a DLC coating result in the necessity of finding solutions in this technological field. Nevertheless, even reducing the residual stress, the solution does not fully resolve the problem by virtue of the fact that the contact interface of the ring with the liner continues to occur by means of a profile which does not ensure, for example, that the cylinder liner does not suffer the wear typical of the solutions of the state of the art.
The document U.S. 2013/0140776 describes a piston ring provided with a DLC coating of the ta-C type having a thickness also exceeding 10 micrometers, the coating containing a finishing layer having a thickness from 1 to 3 micrometers wherein the quantity of sp3 decreases to values of less than 40% with a view to ensuring that a greater quantity of sp2 is present in this second layer for the bedding in of the ring within the cylinder.
Whilst the solutions of the state of the art are concerned with ensuring low friction and high durability, all the solutions found require an interface or presence of elements bringing about the bedding in, such as a softer layer in the sliding region of the ring, or the addition of metals bringing about such effects. Furthermore, there are alternatives in the state of the art, through the reduction of the internal stresses, to maintain the functional integrity of the coating in view of the high internal stresses and high hardness to which the coating is subject.
Independently of the efforts revealed by the documents of the prior art, a solution has still not been found generating excellent results in the finished product, i.e. a piston ring not requiring bedding in within in the respective cylinder liner and which, simultaneously, works with very small clearances.
Consequently, it is necessary to find a sliding element, such as a piston ring, comprising a coating of hard amorphous carbon of the ta-C type provided with a roughness profile capable of bringing about a reduction of the friction in the sliding interface thereof, together with diminishing the wear.